Let's be realistic here. Whether you're hosting a show for millions and millions of people or only a few dozen, you're sitting in a little room with a microphone, maybe some notes, and someone screening calls. The feedback you get as to whether something you're doing works or not comes from (or through) the handful of management types who sign your paycheck. Big or small market, just gabbing into a microphone doesn't make you better at gabbing into a microphone. Good, solid feedback and coaching from someone who knows what they're talking about does help make you better.
But the best that good coaching can do is to polish raw talent. The best coaches/trainers in the world, working with an individual who doesn't have the talent, cannot create a good talk show host. But, given a person blessed with the right way talent, sometimes only minimal coaching is enough to make them into a star.
Talent is a funny thing. You cannot make accurate blanket statements about which group of people have the talent to be talk show hosts and which group doesn't. Pick any possible background your imagination can conceive. It's possible that you might find a good talk show host with that background, and it's possible you won't. Prior career experience is a totally unreliable indicator of whether or not any rookie talk show host will do well.
